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Novel Wideband MIMO Antennas

thatcan

cover

the wholeLTE

spectrumin

Handsets and PortableComputers

Customers’ increasing expectations for speed,bandwidth, and global access is driving the evolutionof wireless broadband technology.Customers wantmore information, such as business andconsumer applications, and entertainment availablethrough their mobile devices, but with greaterspeeds[1]. LTE represents the next big step towardthe 4th generation (4G) of radio technologies whichexpected to increase the capacity and speed ofmobile telephone networks. These expectations put asignificant burden on device performance.

The antenna is becoming an increasingly criticalcomponent for LTE device vendors, In order to meetthe requirements of expanded, high cell capacitydata rates, multiple antenna configurations (MIMO)specified for LTE mobile devices

as smart phones,tablets and notebooks. It is possible that a futureterminal device will have more than 20 antennas tocover

all the important wireless applications [2], inaddition the industry is painfully aware of the issuesthat surround implementing LTE in small mobiledevices with already limited space and extremelyhigh performance expectations. Due to this trend,wideband coverage is a hot issue that has to beaddressed

[3].

The latest GSMA’s wireless intelligence report,predicts that there will be 38 different spectrumfrequency combinations used in LTE deploymentsby 2015. The lack of spectrum harmonizationrepresents

required to include support formany disparate frequencies in their devices

[4].

To satisfy the end user expectation of a globalLTE experience, the FDD/TDD dual mode deviceis highly preferred and a universal RF chipset thatsupports allglobal LTE bands should be required[5].Thus, a compatible wide band LTE antenna isabsolutely needed.

Device makers are finding it difficult to decidewhich bands to prioritize as chipsets and handsetsare developed. The priority is the 800 megahertzband. Most of the investment has been directedtowards that because it is the band that two-thirds ofcurrent LTE users occupy, largely driven by the USnetwork roll outs of Verizon Wireless and AT&T

[6]. This low frequency band means larger antennasin termsof size, which is a challenging issuekeeping

in mind limited size of LTE devices.

However, inEurope and even more so in the Asia-Pacific regionthere are a greater number of LTE bands andcombinations to be addressed.

The roaming point is a critical onebecause operatorsare targeting LTE towards their high valuecustomers, in the initial stages of the market. Those,by definition, are business users with the propensityto roam. Yet, in fragmented regional markets such asEurope, LTE roaming is some way off as operatorswill be providing LTE in different bands and deviceswill need to be able to seamlessly switch betweenthe frequency bands used for LTE in addition to the2G and 3G networks[6].

What is clear is that in order to service the highspending, first wave of LTE users some form ofroaming between LTE spectrum bands and the 2Gand 3G networks will be required. It seems illogicalthat operators will sell the benefits of LTE in theirhome markets but, as soon as the high spending LTEcustomer changes

country, they will only be offered2G or 3G service. For operators, a world LTE deviceat a keen price point is needed—

but it remainssome time away from appearing in the market

[6].

The bigger issue is that operators are running out of3G capacity. They need handsets so they can migrateusers to 4G because it’s more efficient

[6].

The urgent demand for wideband LTE coverage

represents a serious challengefacing

antennavendors

who found themselves in trouble, stuck withthe current passive antenna technology trying toadaptitto serve the wideband demand.Unfortunately, the current technologies were notresilient enough to achieve that purpose.Soantennavendors gave up onit,having

a definitive

belief thatpassive antennas have reached its limits

[7].

Therefore,they adopted the activetunableantennaapproach to fulfill the market need for world LTEdevices, trading the passive antenna simplicity withactive antennacomplexity.

But we can say withconfidence that the passive antenna technology havenot come to an end yet.

First of all, the passive antenna doesn’t have to besupported with RF controlling circuit to do the jobasthe active one, so it will save large space requiredinside the handset to fit both the antenna and the RFcircuit.

The problems of the RF circuit will not end here, asthe additional circuit componentsconnected to theantenna surely will suffer from impedancemismatch

that will be translated into a severe decrease in theefficiency of the antenna.

Inaddition,

Active antenna is power consuming, asthe battery life is significantly decreased.

Finally,active antenna seems to have band limitations too, asthemaximum number of bands that commercial LTEantennascan support is 13 bands out of 35 potentialLTE bands.

On the other hand,Current handset antennatechnology still tide withextended

ground plane’sdilemma, as the antenna is not just the module thatfit under the

ear phone but also includes the largePCB ground planethat must have a minimum size

for the antenna to have an acceptable performance,adding an extra volume counted on the

total size ofthe antenna.

Wide band passive antenna

AMANT

Antennas

is providing the marketwithanovel

antenna technology to solve the problem ofurgent need for universal LTE devices. The newtechnologycan cover all the possible LTE

spectrumbands using only two antennas

with bandwidths of73%

and 75.8% respectivelywithout using anymatching or tuning circuits. The first antenna iscoveringthe low band LTE spectrum

The new antenna technology can be implemented insmart phone handsets, tablets, laptops andnotebooks. The geometry of the antenna is shown inFig.1, whichcan be scaled and optimized for anyapplication or any frequency band. Multiple

antennaconfigurations

is provided

according to the availablespaceinside the device with different sizechoices

of

0.1x0.1x15.6

cm,0.2x0.2x15.6 cmor

0.4x0.4x15.6cm for

the

low band antenna and0.1x0.1x5.55 cm,0.2x0.2x5.55 cmor

0.4x0.4x5.55 cm for the highband antenna.

Fig. 1Geometry of the new wideband antenna

The low bandantenna design is flexible enough

tobe bended and wrapped

in the void

around themobile

chassis

to overcome the antenna lengthproblem and make it fit inside the device so itiscompletely self-contained and does not need anadditional ground plane or any other components.Thus, the new antenna can be mounted anywhereinside or outside any handset because the antennadoes not use a part of the handset as an extendedground plane as usually happens with internalantennas.

Results:

Different

prototypes of the new LTE

antennas

havebeen designed, manufactured and tested.

The resultsof a selected sample antenna configuration will bepresented.

The low band antennaperformancehaving total volume

of:0.4x0.4x15.6 cm=2.496 cm3

is numerically calculated by a software packages thatuses the moment method. It is also measured atIMST antenna labs in Germany.It should be notedthat

the

proposed volume

is thetotal

volume of theantenna because it does not require an additionalground plane or matchingcircuits.

Also, the uncorrelated total radiation patternsdemonstrated in Fig. 18 & Fig. 19 is a furtherevidence of the minimal correlation between primaryand secondary high band MIMO antennas on laptopsand tablets.

Low band & high band antennas isolation

The ultra wide spectrum of LTE requires thepresence of both of the low band & high band in the

device in the same time to cover the whole spectrum.This condition will raise concerns about isolationbetween high & low bands.

We’ve investigated the worst case scenarios for thepresence of low & high band antennas together inlaptops and tablets and

found that the isolation indifferent relative positions is acceptable.

Fig.18

uncorrelated patterns of primary and diversityantennas at phi=0 at 2.755 GHz

Fig.19

uncorrelated patterns of primary and diversityantennas at phi=0 at 2.755 GHz

The first case

scenario

investigatedwasthe low bandantenna parallel to the high band antenna at 10 cmdistance between them.

Asshown in Fig. 20

& Fig.21, the isolation all

over both low and highbandsis

lower than-30 dB.

Fig. 20

Measured &

calculated isolation (S21)

inlow frequency band between low band and highband antennas while parallel to each other

Fig. 21

Measured & calculated isolation (S21) inhigh frequency band between low band and highband antennas while parallel to each other

requires an antenna solution that is able to covermost of LTE bands for global roaming. Bycombining our two wide band antennastogether inonedevice, itwillfulfill

that

need.A schematicdiagramfor our

MIMO wide band antenna solutionis demonstrated in Fig. 44

As shown in Fig. 44 the high band antennas no. 3 &4 are located in a higher

plane above the low bandantennas by 1 mm.

Fig. 44The schematic diagramof MIMO wide band

antenna

solution forLTE smart phones

The biggest concernin this case is the isolationvalues between

the low and high band antennas. Theisolation is tested for this case andresults were

mostly

lower-20 dB forlow band and high bandfrequency spectrum

as

shown in Fig. 45 & Fig 46

respectively.

Fig. 45isolation between each low band and highbandantenna in the low band frequency spectrum

1

2

3

4

Fig. 46 isolation between each low band and highband antenna in the high band frequency spectrum

Dual feed antenna

In some cases the available space inside handsets isvery limitedand the number of MIMO antennas willrepresent a problem. We have developed anotherwide band LTE antenna solution for handsets thatwill save large volume inside the device by dual feedtechnique in one antenna, so the multiband LTE canbe covered only by two antennas as shown in Fig.47.

Fig. 47

The schematic diagram for MIMOdual feedLTE antennas

Thefirst antenna in this solution is the low bandantenna; it is a dual feed for2x2MIMO diversitypurpose. The dual feeds are lapelled 1 & 2in

The low band dual feed antenna has been tested forobtaining return lossdue to the first feed S11 , returnloss due to second feed S22and the isolation

between

them

S21

as shown in Fig. 49.

Fig. 49

s parameters fordual feed low band antenna

The resulted correlation coefficient between the twofeeds is lower than 0.5 for the low band antenna asshown inFig.50.

1

2

Fig. 50

Correlation coefficient between feed 1 &feed 2for low band antenna

The second antenna in the wide band LTE antennasolution for thehandsets

is the high band antennawhich is alsohaving dual feedsfor 2x2 MIMOdiversity. The 2 feeds lapelled 1 & 2 are shown inFig.51.

Fig. 51

The schematic diagram of MIMOhigh

bandantenna solution for LTE smart phones

The high band dual feed antenna has been tested forobtaining return loss due to the first feed S11 , returnloss due to second feed S22 and the isolationbetween them S21 as shown in Fig.52.

The resulted correlation coefficient between the twofeeds is lower than 0.6 for the lowband antenna asshown in Fig. 53.

Fig. 52

s parameters for dual feed high band antenna

Fig. 53

Correlation coefficient between feed 1 &feed 2 forhigh

band antenna

Improving LTE antennaperformance

Asshown above, our wide band

antenna

can covermost of the LTE frequency spectrum. However, thesmall space availablefor the antenna inside thedevices makes it a compromise process:

To fittheantennainside suchsmall space makesus

acceptlower

but acceptable

efficiencies.As the availablespace

is larger it would bedesirable &easy toincrease the efficiencyby increasing the width andthickness of the antenna as much as possible. A newantenna has been developed for this purposehavinga total volume of 1.3x0.4 x14.5=7.54 cm3.Fig. 54

shows the return loss of the increased size antenna,which is lower than-8

dB over most of the band1

2

having minimum value of-5dB.

The total efficiencyshown inFig. 54demonstrates minimum efficiencyof 70% and reaches its maximum at96%.